1. Field of the Invention
My present invention relates generally to electrostatic chucks. More particularly, my invention relates to ail electrostatic chuck which uses A.C. clamping voltages and to a method for clamping an article such as a semiconductor wafer to an electrostatic chuck with (1) a large ratio of clamping force to A.C. clamping voltage, (2) suppression of the decay of the clamping forte while the clamping voltage is being applied and (3) substantially instantaneous decay of the remnant clamping force when the clamping voltage is removed; and (4) without substantial vibration of the wafer.
2. Description of the Related Art
Electrostatic clamps or chucks have been used to hold wafers on pedestals, electrodes, etc., for processing in semiconductor wafer fabricating systems, In addition, robots which incorporate electrostatic clamping have been used to load and unload wafers in such systems.
Existing electrostatic clamps or chucks include uni-polar electrode designs which use AC (alternating current) or DC (direct current) excitation. A single electrode, separated from the wafer by a dielectric material, is biased with a grounded AC or DC power supply. The wafer acts as the second electrode and, together with the intervening dielectric and the first electrode, forms a parallel plate capacitor. In a plasma processing reactor, the plasma references the wafer to system ground. Clamping force per volt of applied clamping voltage is high. However, a net charge develops on the wafer and on the chuck, which causes several problems.
First, the clamping force F, cannot be instantaneously turned off. A significant remnant remains, typically for as long as about 30 to 60 seconds after the clamping voltage is removed. As a consequence, once the clamping voltage is removed to permit wafer transfer to or from the pedestal or blade, one must either wait for the remnant clamping force to decay or apply a potentially excessive lifting force to remove the wafer. In short, the choice is between throughput-decreasing delay and potential damage to the wafer.
Also, the net charging of the wafer may damage sensitive integrated circuit devices formed on the wafer.
Alternative electrostatic clamp designs use a pair or multiple pairs of isolated bipolar electrodes which are buried beneath a dielectric layer, thereby isolating the electrodes from the wafer. AC or DC excitation is used. Either AC or DC excitation causes charge separation and/or polarization in the wafer without net charging. However, DC excitation devices suffer from the above-discussed decay of force with time to some low value, as well as the above-discussed turn-off delay when the DC clamping voltage is removed. AC designs experience vibration during the application of the AC clamping voltage. Also, unbalanced AC designs may suffer both clamping force decay and turn-off delay. FIG. 1 depicts the typical decay of the clamping force, F.sub.c, which occurs despite the continued application of the 1000 volt DC clamping voltage that generates F.sub.c. It is not unusual for F.sub.c to decay to about 25% of its original value within 20-30 seconds.
U.S. Pat. No. 4,184,188 illustrates a multiple electrode pair electrostatic wafer clamp and the general principles of electrostatic wafer clamps. The disclosed clamp includes a blade with a multiplicity of parallel interleaved positive and negative paired electrodes arranged on the blade surface. A dielectric layer is disposed over the positive and negative electrodes, forming a multiplicity of capacitors on the blade surface. During electrode energization by a D.C. (direct current) voltage, fringing electrostatic fields are generated between the positive electrodes and the negative electrodes of each capacitor pair. When a wafer is placed over the charged electrodes, the fringing electrostatic forces between the positive and negative electrodes pass through the wafer, creating a clamping force across the wafer and the electrodes by separating charges, so that negative charges in the wafer collect over the positively charged electrodes and positive wafer charges collect over the negating charged electrodes. The clamping force is directly proportional to this electrostatic force acting upon the wafer.
U.S. Pat. No. 4,733,632 discloses a representative prior art wafer transfer apparatus which uses a single electrode pair. The apparatus includes a movable arm, a D.C. electrostatic chuck mechanism having a vertically moving support member attached to the moving arm, and a mechanism for moving the support member. Wafer transport is accomplished by positioning the electrostatic chuck above the wafer to be transferred. The electrostatic chuck or robot has two semi-circular electrodes, which are electrically isolated from one another by an insulating film formed over each electrode and are arranged to fit over the periphery of the top surface of the wafer to be clamped. (The periphery of wafers generally is not processed, and therefore provides a clamping surface.)